Abstract:

Photoneutron contamination produced during high-energy, external beam radiotherapy can contribute to total patient dose. This aspect of unwanted dose is not currently evaluated for patients undergoing radiation therapy. While multiple studies have examined the neutron dose from beams with energies above 10 MeV, research assessing neutron dose from a 10 MV beams is sparse. Additionally, no studies have assessed the neutron dose from volume modulated arc therapy (VMAT) or compared to that of intensity modulated radiation therapy (IMRT). A series of physical measurements were performed using neutron bubble dosimeters to determine neutron dose per Gy photon dose for field sizes 0x0 cm², 5x5 cm², and 10x10 cm² on a Varian Trilogy linac operating at 10 MV. These measurements were compared to photoneutron production simulated with a simplified model of the linac head using MCNP5. The simulations predicted within 6% of the measured dose for the 5x5 field and within 3% for the 10x10 field, however over predicted over 600% for the closed field. This indicates that the simplified model is not appropriate for small field neutron dose calculations but may be a good starting point for more complete models.
Additional measurements were collected for clinically representative IMRT and VMAT plans designed to treat the same prostate tumor. Total neutron dose was higher 14.6% for the IMRT plan, which was less than expected based on the total delivered monitoring units and may be related to different average field sizes for the two treatments. This under-prediction of neutron dose suggests that neutron dose estimates should be independently assessed for the different modalities.